JSFiddle - React, Tailwind, and code Playground

JavaScript

var sentence1 = "This is the original sentence.", // original text
    sentence2 = "This is teh originel sentence.", // what the user typed
    length = sentence1.length,
    actualScore = length, // start with full points
    i = 0;

while(i<length){
    if(sentence1[i]!==sentence2[i]){
        actualScore--; // subtract 1 from actual score
    }
    
    i++; // move to the next index
}

alert("'sentence2' is "+Math.round(100*(actualScore/length))+"% accurate");